Piezoelectric relay with a piezoelectric longitudinal effect actuator

ABSTRACT

A miniature relay having dimensions in the order of 30 mm×24 mm×4 mm is driven by an expansion type piezoelectric actuator element. The actuator drives an interconnected series of lever arms which rest on a mounting block. The actuator rests on an anchor block. The coefficients of thermal expansion for the actuator, the mounting block and the anchor block are mutually cancelling.

BACKGROUND OF THE INVENTION

This invention relates to a piezoelectric relay which uses apiezoelectric actuator as a contact driving source and, moreparticulary, to a piezoelectric relay having a multilayer piezoelectricactuator comprising a plurality of stacked piezoelectric ceramiclaminates.

Almost all the relays which have been put into practical use areelectromagnetic relays using exciting coils as the contact drivingsource. Even in the age when semiconductor technology has progressedremarkably, the demand for electromagnetic relays continues to increasein various fields because they endure higher voltage and have betterswitching characteristics, as compared with semiconductor switchingelements. However, those relays which are equipped with the excitingcoils cannot avoid having a large power comsumption, high heatgeneration, and large size fabrication. Also, the relays tend togenerate magentic fields which affect adjacent circuit elements, such asthose elements which are mounted on a printed wiring board.

Recently, in order to solve these and other problems ofelectromechanical relays, efforts have been made to developpiezoelectric relays which use a piezoelectric actuator, in place of theexciting coils, as the contact driving source. As is well known, apiezoelectric actuator is a transducer for converting between electricalenergy and mechanical energy, the actuator physically deforms to cause adisplacement responsive to an application of voltage onto piezoelectricceramic laminates. The piezoelectric actuators are classified into thepiezoelectric transverse effect type or bimorph type whose laminates arebendably displaced due to mechanical strain occurring in the verticaldirection in response to an electrical field and the piezoelectriclongitudinal effect type or multilayer type whose laminates areexpandably displaced due to the mechanical strain occurring in thedirection which is parallel to the electrical field.

The relays using the piezoelectric transverse effect type or bimorphtype actuators as the contact driving source have been shown in thestructures disclosed, for example, in U.S. Pat. Nos. 4,403,166 and4,425, 524. In these relays, a bimorph type actuator can give a largedisplacement to a movable contact, but bending displacement which iscaused by the expansion of two laminates consumes energy and inveitablylowers the energy conversion efficiency. If the relays are required tobe miniaturized to be mounted with other circuit elements on a printedwiring board, they cannot apply sufficient contact pressure between amovable contact and a stationary contact. When the driving voltage isapplied continuously to the bimorph type actuator, it becomes impossibleto stably open and close the contacts for a long time of period, becausethere is a change in the displacement charcteristics. Therefore, thepiezoelectric relays using the bimorph type actuators have heretoforenot been put into practical use.

The relays using a piezoelectric longitudinal effect type actuator asthe contact driving source, on the other hand, have a disadvantagebecause the degree of obtainable displacement is extremely small ascompared with the relays using th bimorph type actuator. Thedisadvantage can be solved by increasing the voltage applied on theactuator to compensate for the electrical field intensity. However, theapplicable voltage is necessarily limited within a certain range becausethe driving control circuit of the actuator has a low voltag resistance.However, such a limitation imposed on the applied voltage becomes abottleneck problem in the practical use of the relays with this type ofthe actuators.

U.S. Pat. No. 4,454,442 discloses an example of a piezoelectric relaywhich attempts to solve the bottleneck problem. The relay disclosedtherein has a structure which enables a minute displacement occurring inthe longitudinally expandable piezoelectric body (actuator) to bemagnified by a single resilient elongated member made of a dielectricmaterial or by a mechanical amplification member so as to make connectbetween contacts. More particularly, in this type of relay structure, aminute displacement (several μm) of the piezoelectric body should bemagnified several tens of times by bending the single resilient memberto gain a sufficient distance (e.g. 0.55 mm) for moving the contacts.However, it is almost impossible to magnify the displacement at a highprecision, in the case of a miniaturized relay.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide apiezoelectric relay which has a piezoelectric longitudinal effectactuator as a contact driving source.

Another object of the present invention is to provide a piezoelectricrelay which is equipped with a multi-stage displacement magnifyingmechanism for effectively increasing the displacement without requiringan increment increase of the voltage applied on the actuator.

Still another object of the present invention is to provide apiezoelectric relay which can be operate stably irrespective of anythermal expansion of structural members which may occur due to ambienttemperature variation.

Further object of the present invention is to provide a piezoelectricrelay of a small size and particularly of a thin type which can bemounted on a printed wiring board together with other circuit elements.

Still further object of the present invention is to provide apiezoelectric relay with structural members which can be easilyassembled at a high precision.

According to one aspect of the invention, a piezoelectric relaycomprises a base frame made of a synthetic resin having a base plate. Afirst block has a first arm and a second arm which extend in parallelwith each other and a connection channel which connects the arms, all ofwhich are fixed on the base plate. A second block engages the connectionchannel of the first block. The relay further includes a piezoelectricactuator with one end portion extending in the axial direction whileabutting on the second block and the other end portion having a drivingchip, which is arranged between the first arm and the second arm of thefirst block. The actuator is expansion-displaced in the axial directionin response to an applied voltage. The relay still further includesdisplacement magnifying means having an input section which responds tothe displacement of the actuator, and a first-stage magnifying sectionand a second-stage magnifying section which sequentially magnify theoutput displacement of the input section. A movable resilient member isdriven by the magnifying means to establish a connection between amovable contact and a stationary contact. The relay also is charcterizedin that the relationship is satisfied as Δl₂ =Δl₃ -Δl₁ wherein thedimensional variations caused by the various temperature variationsamong the actuator, the first block and the second block are denotedrespectively as -Δl₁, Δl₂, and Δl₃.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of th invention may be fullyunderstood from the following detailed description and the accompanyingdrawings in which:

FIG. 1 is a perspective view of the first embodiment of the invention;

FIG. 2 is an exploded perspective view of the relay shown in FIG. 1;

FIG. 3 is a plan view for describing the contact driving mechanism andthe displacement magnifying mechanism of the relay shown in FIGS. 1 and2;

FIG. 4 is an exploded perspective view of a first modification to thedisplacment magnifying mechanism shown in FIGS. 1, 2, and 3;

FIG. 5 is a perspective view of a second modification to thedisplacement magnifying mechanism shown in FIGS. 1, 2 and 3 and of aslightly modified contact driving mechanism; and

FIG. 6 is a perspective view of a second embodiment of the invention.

In the drawings, the same reference numerals denote identical structuralelements.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGS. 1 and 2, a piezoelectric relay in the firstembodiment of the invention comprises a base frame 1, a terminalassembly 2, a contact driving mechanism 3, and a displacement magnifyingmechanism 4. The base frame 1 is made of synthetic resin and comprises abase plate 10, erect walls 11, 12, 13 and 14 and a rectangular window15.

The base plate 10 is provided with a pair of projections 16. Theterminal assembly 2 has a plurality of terminals 20, 21, 22, 23 and 24which penetrate through the wall 11 of the base frame 1 and are fixedtherein. The terminals 20 and 21 are used to supply electrical voltageto a piezoelectric actuator, which will be described hereinafter indetail. The wires connected to the terminals 20 and 21 pass through apair of tunnels 17 and 18 which are bored or otherwise formed in thebase plate 10.

One end of a movable contact spring 26 is welded to the terminal 22while the spring 26 is fixed to a movable contact 25 on the other end.Stationary contact terminals 28 and 29, each with a fixed stationarycontact 27, are welded to the terminals 23 and 24 in a manner to opposethe spring 26.

The contact driving mechanism 3 has a base block 30 with a substantiallyU-shaped cross section. The block 30 has a first arm 301 and a secondarm 302 which extend in parallel to each other, but which have differentlengths. A connection channel 303 connects the arms 301 and 302. Thechannel 303 has a guide groove 304 with which a push block 31 engages.One end 311 of the push block 31 is positioned in a space between thefirst arm 301 and the second arm 302 to support a piezoelectric actuator32. When the mechanism 3 is pre-assembled with the displacementmagnifying mechanism 4, the push block 31 is used to position theactuator 32 against the magnifying mechanism 4 with a very highdimensional precision. After this arrangement is completed, the pushblock 31 is welded to the base block 30.

The second arm 302 of the block 30 is bored or otherwise formed to havea pair of holes 305 which receive the pair of projections 16 of the baseframe 1. The arm 302 is further provided with an extension portion 307having a stopper 306. The function of the stopper 306 will be describedherinafter together with the description of the magnifying mechanism 4.

In this embodiment, a piezoelectric longituindal effect type actuator (amulti-layer piezoelectric actuator) is used as the piezoelectricactuator 32. The actuator 32 comprises 128 ceramic laminates of Pb(Ni1/3, Nb2/3)O₃ --PbTiO₃ --PbZrO₃ which are stacked with internalelectrodes. The thickness of each of the laminates is 60 μm. Theactuator 32 has the length of 9 mm, incuding internal electrodes, thewidth of 3 mm, and the height of 2 mm in external dimension. Theactuator 32 causes minute displacement of 7.8 μm and generates force of21,000 g with a driving voltage of DC 60 V and has the energy conversionefficiency of 49%. The piezoelectric actuator of this type can befabricated by using the technologies described in (1) a paper titled"Internal Electrode Piezoelectric Ceramic Actuator", S. Takahashi et al,published in the Ferroelectrics, 1983 Vol. 50, pp. 181-190, and (2) apaper titled "Piezoelectric Ceramic Tiny Actuators", A. Ochi et al,published in the Proceeding of the 3rd Sensor Symposium, 1983, pp.261-265. One end of the actuator 32 has a driving or push chip 320 witha sharp tip angle.

The displacement magnifying mechanism 4 magnifies the minutedisplacement (7.8 μm) caused by the piezoelectric actuator 32 of thecontact driving mechanism 3 about seventy times (0.55 mm) before ittransmits the displacement to the contact spring 26. The mechanism 4comprise an input section which receives as its input the displacementand force applied from the actuator 32 via the push chip 320. Mechanism4 also has a first-stage and a second-stage magnifying sections whichmagnify the displacement and the output force from the input section.

The input section comprises a first resilient hinge lever 40 which is onthe first arm 301 of the base block 30 and a first non-resilient (hard)lever 41 which is fixed on the lever 40 on one end thereof. The arm 41is pressed by the push chip 320 at about the center of the longitudinallength. The first-stage magnifying section comprises a second resilienthinge lever 42 fixed on the other end of the first hard lever 41, athird resilient hinge lever 43 fixed on one end thereof to the secondarm 302 of the base block 30 and a second hard lever 44 fixed at one endthereof on the second and the third levers 42 and 43.

More specifically, the lever 44 is cut to have a step 45. The levers 42and 43 are fixed respectively at two different locations which aredivided by the step 45. The second-stage magnifying section comprises afourth resilient hing lever 467 fixed on the other end of the secondhard lever 44, a fifth resilient hinge lever 47 fixed at one end of theextension portion 307 of the base block 30, and a third hard lever ordriving lever 49 fixed on the hinge levers 46 and 47. A driving chip 48made of a synthetic resin is fixed on the other end. The levers 41, 42,43, 44, 45, 46, 47, and 49 are welded to the base block 30 at thepre-assembling stage.

The magnified output displacement of the mechanism 4 of theabove-mentioned structure can theoretically be obtained by amultiplication of the input displacement with the respective leverratios. In reality, however, the output displacement and the outputforce are varied by the combination of dimensions of the structuralmembers (i.e., levers 41, 42, 43, 44, 45, 46, 47 and 49) of themagnifying mechanism 4. The inventors have obtained optimal dimensionsof the structural members for driving the relay contact by using thefinite element method (FEM) of analysis. In designing the highefficiency displacement magnifying mechanism 4 by means of the FEManalysis, the most important factor is the length among the point offorce, the fulcrum, the point of the application of each hard lever(namely lever ratio) and the spring stiffness of each hinge lever.

FIG. 3 shows an example of the mechanism 4 wherein the dimensions of themembers are obtained by the FEM analysis. Referring to FIG. 3, themechanisms 3 and 4 are described in more detail. Each of the structuralmembers of the mechanisms 3 and 4 is preferably made of metal so that itcan effectively respond to an input displacement (δ1) caused by thepiezoelectric actuator 32 in the magnifying mechanism 4 and can magnifythe displacement (δ1) by about 70 times to obtain displacement (δ2).However, due to the difference in thermal expansion coefficients causedby ambient temperature between the actuator 32 and other metal members,the displacement (δ1) from the actuator 32 may not effectively betransmitted. In the worst case, an air gap may be generated among theactuator 32 and other metal members which completely incapacitates theactuator 32. In order to solve such problems, the base block 30 and thepush chip 320 of the contact driving mechanism 3 are made of invar (analloy of Fe-Ni 36) while the push block 31 is made of stainless steel.All the levers 41 through 47 and 49 of the magnifying mechanism 4 aremade of invar.

If a piezoelectric relay is used at about 120° C. or below, the actuator32 shrinks by a minute dimension -Δl₁ as a normal scope of thepermissible operational temperature ranges from -20° C. to +80° C. andthe thermal expansion coefficient α of the actuator 32 is about -6×10⁻⁶/°C. The base block 30 and the magnifying mechanism 4, which is made ofinvar of α=1.2×10⁻⁶ /°C., expand by a minute dimension Δl₂ while thepush block 31 made of stainless steel of α=17×10⁻⁶ /°C. expands by arelatively large dimension Δl₃. The dminesional variation caused bytermperature variations can terefore be compensated if the relation Δl₂=Δl₃ -Δl₁ is satisfied.

The fourth and the fifth resilient hinge levers 46 and 47 and the thirdhard lever 49 may be made of stainless steel, since the levers 46, 47and 49 are not greatly related to the temperature compensation. Asabove-described, the push block 31 is arranged serially to the actuator32 to compensate for the difference in the thermal expansioncoefficients α between the actuator 32 of the mechanisms 3 and 4,thereby realizing a piezoelectric relay which can operate stablyirrespective of the variation in the ambient termperature.

Referring now again to FIGS. 1 and 2, a stopper 306 is provided on theextension portion 307 of the contact driving mechanism 3. Stopper 306abuts against the fifth hinge lever 47 supporting the third hard lever49 of the magnifying mechanism 4. This arrangement not only enables aneffective transmission of the magnified displacement (δ2) from the lever49 to movable contact spring 26, but also restricts vibration whichmight be caused when the lever 49 releases its position to perform ahigh speed switching operation.

The piezoelectric relay according to the above-mentioned embodiment ofthe invention is usually housed in a cover (not shown). If necessary, itmay be hermetically sealed with a plastic resin. The terminals 20, 21,22, 23 and 24 led out of the base frame 1 may be bent if necessary. Therelay has the length of 30 mm, the width of 24 mm and the height(thickness) of 4 mm in external dimensions and has the weight of 6.5 g(grams). Further, the nominal drive voltage is DC 60 V and, whenoperating time is 5 mS, the maximum consumption current is 3 mA inoperating stage.

Referring to FIG. 4, a first modification of mechanism 4 shown in FIGS.1, 2 and 3 includes a magnifying mechanism 400 formed by punching outfirst, second, third, fourth and fifth resilient hinge levers 401, 402,403, 404, and 405 from a sheet of invar metal, bending the levers inpredetermined forms, and welding first, second and third hard lever 406,407 and 408, respectively, to connection channels 409 which connect thehinge levers 401 to 405 to one another. The stiffness of the hard levers406, 407 and 408 can be reinforced by this arrangement. The numbers ofboth the welding portions and the assembling structural members can bereduced to simplify the assembly process.

FIG. 5 shows a second modification to the mechanism 4 and a modificationto the driving mechanism 3. As compared with the structure shown inFIGS. 1, 2 and 3, these modifications provide a stopper 3060 directly onthe side wall 3020 of the second arm 302. The extension portion 307 isdeleted from the base block 30 of the mechanism 3. Further, the stopper3060 abuts on the place where the fourth hinge lever 46 and th thirdhard lever 490 are fixed. The lever 490 extends in the verticaldirection to the fourth and fifth hinge levers 46 and 47. The functionof the stopper 3060 in the mechanism 3 and the dispalcement magnifyingfunction of the mechanism 4 are similar to the first embodiment.

Referring to FIG. 6, a piezoelectric relay in the second embodiment ofthe invention comprise a base frame 1, a terminal asembly 2, a contactdriving mechanism 3, and displacement magnifying mechanism 4.

In the relay all the terminals 200 through 213 are implanted on a baseplate 100 of the base frame 1. Erect walls 110, 120, 130 and 140 of theframe 1 are on four sides of the plate 100 respectively. Movable contactsprings 214, 215, 216 and 217 having movable contacts are welded to theterminals 202, 203, 204 and 205. The terminals 206 to 213 havestationary contacts. Four movable contact springs 202 to 205 are drivensimultaneously by a driving chip or ladder 480 of a comb-like form fixedon the third hard lever 49. Therefore, the relay has four-transfer typecontacts.

Other structural members are basically similar to those of the firstembodiment. However, the size of the piezoelectric actuator 32 and themechanism 4, of this embodiment, should be slightly increased in orderto cope with the increase of the contact loading. The size can beoptimized in accordance with the above-mentioned design method. Anycover (not shown) may be used so long as if fits with the shape of anopening of the base frame 1.

Other alternatives and modifications to the above-mentioned embodimentscan be made within the scope of the invention defined by the appendedclaims.

What is claimed is:
 1. A piezoelectric relay comprising:a base framemade of synthetic resin and having a base plate; a first block having afirst arm and a second arm extending in parallel with each other, and aconnection channel fixed on said base plate for inter-connecting saidarms; a second block engaged with said connection channel of said firstblock; an elongated piezoelectric actuator having one end portionabutting against said second block and extending in the axial directionthereof, said actuator having a driving chip on its other end portionthereof, said actuator being arranged between said first arm and saidsecond arm of said first block, and being expansion-displaced in saidaxial direction in response to applied voltage; displacement magnifyingmeans having an input section for receiving as an input the displacementfrom said actuator, and a first-stage magnifying section and asecond-stage magnifying section for sequentially magnifying the outputdisplacement of said input section; said input section of saiddisplacement magnifying means comprising a first resilient lever havingone end which is fixed to said first arm of said first block, a firstnon-resilient lever which is fixed at one end portion thereof to theother end portion of said resilient lever, said first non-resilientlever abutting against said driving chip of said actuator atsubstantially a center portion thereof; said first-stage magnifyingsection comprising a second resilient lever which is fixed at one endportion thereof to the other end portion of said first non-resilientlever, a third resilient lever which is fixed at one end portion thereofto said second arm of said first block, a second non-resilient leverwhich is fixed at one end portion thereof to the other end portion ofsaid second and third resilient levers; said second-stage magnifyingsection comprising a fourth resilient lever which is fixed at one endportion thereof to the other end portion of said second non-resilientlever, a fifth resilient lever which is fixed at one end portion thereofto the other end portion of said second arm, said second arm of saidfirst block having a stopper which abuts on at least one of said fourthand fifth resilient levers of said magnifying means, a thirdnon-resilient lever which is fixed at one end portion thereof to theother end portion of said fourth and fifth resilient levers; and amovable resilient member driven by said magnifying means to connectbetween a movable contact and a stationary contact; wherein Δl₂ =Δl₃-Δl₁ is satisfied where the dimensional variations caused by thetemperature variation among said actuator, said first block, and saidsecond block are -Δl₁, Δl₂, and Δl₃, respectively.
 2. The piezoelectricrelay as claimed in claim 1, wherein said first block is made of invar.3. The piezoelectric relay as claimed in claim 2, wherein said secondblock is made of stainless steel.
 4. The piezoelectric relay as claimedin claim 1, wherein said first, second, third, fourth and fifthresilient levers and said first, second and third nonresilient levers ofsaid magnifying means are made of invar.
 5. The piezoelectric relay asclaimed in claim 1, wherein said first, second and third resilientlevers and said frist and second non-resilient levers of said magnifyingmeans are made of invar; and said fourth and fifth resilient levers andsaid third non-resilient lever are made of stainless steel.
 6. The relayof claim 1 and a plurality of movable contacts springs, and ladder meanscoupled between said movable resilient driven member and said movablecontacts springs.